1. Field of the Invention
This invention relates to a radiation image diagnostic apparatus for reproducing and displaying a radiation image of an object on a display device.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, low graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
When a radiation image containing therein an image which can be an image of an abnormality is reproduced, for instance, on a CRT as a visible image in the aforesaid systems using an X-ray film or a stimulable phosphor sheet, sometimes it is difficult to determine what the abnormality is. In order to overcome such difficulties, there has been proposed, in U.S. patent application Ser. No. 260,349, a radiation image diagnostic apparatus in which a reference image can be displayed together side by side with the radiation image to be viewed. Further, in the patent publication, there is disclosed a radiation image diagnostic apparatus which, when viewing an X-ray image of the chest containing therein an image of pneumoconiosis, automatically selects a reference X-ray image which is the most similar to the X-ray image to be viewed from a number of reference X-ray images and displays the most similar reference X-ray image together with the X-ray image to be viewed.
However, it is very difficult to select the most similar reference X-ray image with a high accuracy except the case where the image of the abnormality to be viewed clearly appears over a wide range of the X-ray image like pneumoconiosis or where the purpose of the diagnosis is limited in advance to a particular abnormality such as pneumoconiosis. Accordingly, when it is difficult to determine the kind of abnormality whose image is contained in the X-ray image while what abnormality is contained in the X-ray image to be viewed has not been informed in advance, it is necessary to display in sequence X-ray images containing therein images of various abnormalities whose kinds have been known side by side with the X-ray image to be viewed.